1. Field of the Invention (Technical Field)
The present invention relates to methods of deposition of thin films for micromachining and diffusion barrier applications, especially for aluminum and copper metallization schemes.
2 Background Art
There is great interest in the deposition of diffusion barriers for both aluminum and copper based metallization schemes. Ternary, refractory based diffusion barriers deposited by physical vapor deposition are excellent diffusion barriers to both copper and aluminum. However, chemical vapor deposition (CVD) has several advantages over physical vapor deposition techniques for industrial applications.
Unlike present titanium based systems which can only be deposited at low temperatures using metalorganic precursors, it is preferred to employ a system in which all the precursors are readily available and accepted by the integrated circuit community.
The present invention has produced a means of depositing recently identified diffusion barrier materials (W--Si--N and W--B--N) by means of CVD. W--Si--N has previously been identified as a potentially useful diffusion barrier and the CVD technique is widely used in the semiconductor industry. However, no one has heretofore demonstrated the ability to use the CVD deposition technique to deposit W--Si--N or W--B--N films or the chemistries used to achieve this end. Properties of the present deposition technique that make it uniquely suited to semiconductor processing are the low deposition temperatures, appropriate deposition rates, and wide process windows.
The semiconductor industry primarily uses three techniques for the deposition of thin films: evaporation, sputtering, and CVD. The following will briefly describe the relative strengths and weaknesses of each of these techniques. Evaporation is a simple and straight-forward technique that allows the deposition of elemental metals. Unfortunately, because evaporation only works reproducibly with elemental films and cannot reliably produce films with more than one constituent (like W--Si--N), it is being used for fewer and fewer applications in the semiconductor industry. Also, evaporation does not provide conformal film coverage over sharp steps (similar to the way snow covers the ground and tops of buildings, but not building walls).
The second technique, sputtering, can produce films with more than one constituent (like W--Si--N), but the conformality remains poor. Sputtering is more complicated than evaporation and, therefore, more expensive, although sputtering allows greater flexibility than evaporation in the composition of the deposited material. It is because of this ability to adjust the composition of the deposited films that sputtering is the most widely used technique for depositing metal films for ULSI applications. Unfortunately, sputtering produces films with poor conformality and there is wide belief that because of this problem sputtering will not be able to provide films for future generations of ULSI devices.
The most complicated of the typical deposition techniques is CVD, although this technique can produce conformal films. In this technique reactive gases are passed over the sample where they react to form a solid film on the sample and volatile by-products that are pumped out of the reaction chamber. The complications caused by the reactive gases are many, including the safety issues involved with handling and disposing of the gases, the requirement for precise flow control of the gases over the sample, and corrosion of parts that come in contact with the gases. Because of these problems CVD is more difficult and expensive than evaporation or sputtering and is used only in applications where good film conformality is required, such as the deposition of SiO.sub.2 over metal lines.
Diffusion barriers are used extensively in the semiconductor industry as part of the metallization scheme for ULSI devices. The diffusion barrier that is most widely used today is sputtered TiN, a polycrystalline material that must be 50 to 100 nm thick in order to provide sufficient barrier properties for present ULSI applications. In the future, thinner diffusion barriers will be required that can provide sufficient barrier properties at thicknesses of 10 to 30 nm (considerably less than the thickness of 30 atoms). These new barrier requirements will mean that TiN cannot be used as the diffusion barrier for future ULSI devices and will also mean that variations in the film thickness (poor conformality) will not be acceptable.
A new class of diffusion barriers has been identified by researchers at the California Institute of Technology (Caltech). This class is of amorphous refractory ternary films, such as W--Si--N, Ti--Si--N, and Ta--Si--N. These barriers have been shown to be effective at thicknesses of approximately 10 nm, the thickness that the semiconductor industry needs for future applications. However, the barrier materials developed at Caltech have been deposited by sputtering and not CVD.
The present invention is of CVD deposition techniques for W--Si--N and W--B--N. The chemistry allows the deposition of W--Si--N and W--B--N at temperatures, times, etc., that are attractive to the semiconductor industry, and circumvents the limitation of the prior sputtering deposition techniques of these films.
References related to the present invention include: U.S. Pat. No. 5,278,100, to Doan et al. (Ti--Si--N deposition by CVD); U.S. Pat. No. 5,369,300, to Heideman et al. (amorphous W--Si deposition by sputtering); U.S. Pat. No. 5,376,405, to Doan et al. (Ti--Si deposition by CVD); U.S. Pat. No. 5,416,05, to Kauffman et al. (CVD of TiN); U.S. Pat. No. 5,254,499, to Sandhu et al. (CVD of TiN); U.S. Pat. No. 5,378,501, to Foster et al. (CVD of TiN); U.S. Pat. No. 4,640,004, to Thomas et al. (use of refractory nitrides as blanket layer); Reid et al., "W--B--N Diffusion Barriers for Si/Cu Metallizations", Thin Solid Films 262:218 (January 1995) (sputtered W--B--N); Lin et al., "TiWN Schottky Contacts to n-Ga.sub.0.51 In.sub.0.49 P", Japanese Journal of Applied Physics 33:4546-49 (August 1994) (sputtering of Ti--W--N); Eizenberg et al., "TiCN: A new chemical vapor deposited contact barrier metallization for submicron devices", Applied Physics Letters 65:2416-18 (November 1994) (CVD of Ti--C--N); Reid et al., "Evaluation of amorphous (Mo, Ta, W)--Si--N diffusion barriers for &lt;Si&gt;.vertline.Cu metallizations", Thin Solid Films 236:319-24 (January 1993) (sputtering of (Mo, Ta, W)--Si--N); Kolawa et al., "Amorphous W.sub.40 Re.sub.40 B.sub.20 diffusion barriers for &lt;Si&gt;/Al and &lt;Si&gt;/Cu metallizations", Thin Solid Films 236:301-05 (January 1993) (sputtering of W--Re--B); Reid et al., "Thermodynamics of (Cr, Mo, Nb, Ta, V, or W)--Si--Cu ternary systems", Journal of Materials Research 7:2424-28 (September 1992) (thermodynamic study of metal-Si--Cu ternary systems); and Kolawa et al., "Sputtered Ta--Si--N Diffusion Barriers in Cu Metallizations for Si", IEEE Electron Device Letters 12:321-23 (June 1991) (sputtered Ta--Si--N).